An X-ray image diagnosis apparatus is an apparatus for obtaining tomographic images (slice images) of an examinee such as a patient by causing an X-ray irradiator to irradiate the examinee with X-rays, causing an X-ray detector to detect an amount of X-rays transmitted through the examinee, and performing reconstruction processing on X-ray transmission data which is based on the detected amount of X-rays. As an example of such an X-ray image diagnosis apparatus, an X-ray computed tomography (CT) apparatus has been developed. The X-ray CT apparatus is configured to image an examinee using an X-ray irradiator and an X-ray detector located opposite each other with the examinee in between, while rotating them about the body axis of the examinee.
In such an X-ray image diagnosis apparatus, generally, in order to set a scan field (imaging range) before imaging such as multislice scan or helical scan, scanograms are taken and collected by imaging the examinee without rotating the X-ray irradiator and the X-ray detector. Some X-ray image diagnosis apparatuses are provided with a function of calculating an optimum tube current supplied to the X-ray irradiator by using the scanograms (such a function is called, for example, Real-EC). With this function, an X-ray amount appropriate for the body thickness of each part of the examinee is automatically computed so as to allow detailed control of the X-ray amount for each rotation of the imaging device. Thereby, while maintaining the high image quality, unnecessary radiation exposure is suppressed to achieve reduction in radiation exposure.
However, the above function performs the computation assuming that the examinee is located at the center of an effective field of view (FOV). Actually, it is difficult to locate the examinee at the center of the effective field of view, and the positioning of the examinee is dependent on an operator. Thus, the reproducibility of examination is degraded.